moved baseline alignment to a layout spec.

AsyncDisplayKit/Layout/ASStackTextLayoutSpec.h

@@ -1,35 +1,48 @@
-//
-//  ASStackTextLayoutSpec.h
-//  AsyncDisplayKit
-//
-//  Created by ricky cancro on 8/19/15.
-//  Copyright (c) 2015 Facebook. All rights reserved.
-//
+/*
+ *  Copyright (c) 2014-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
 
 #import <AsyncDisplayKit/ASStackLayoutSpec.h>
 
-/** Orientation of children along cross axis */
 typedef NS_ENUM(NSUInteger, ASStackTextLayoutBaselineAlignment) {
-    ASStackTextLayoutBaselineAlignmentNone,
-    /** Children align along the first baseline of the stack. Only available for horizontal stack nodes */
-    ASStackTextLayoutBaselineAlignmentFirst,
-    /** Children align along the last baseline of the stack. Only available for horizontal stack nodes */
-    ASStackTextLayoutBaselineAlignmentLast,
+  /** No baseline alignment. This is only valid for a vertical stack */
+  ASStackTextLayoutBaselineAlignmentNone,
+  /** Align all children to the first baseline. This is only valid for a horizontal stack */
+  ASStackTextLayoutBaselineAlignmentFirst,
+  /** Align all children to the last baseline. This is useful when a text node wraps and you want to align
+   to the bottom baseline. This is only valid for a horizontal stack */
+  ASStackTextLayoutBaselineAlignmentLast,
 };
 
 
 typedef struct {
-    /** Specifies the direction children are stacked in. */
-    ASStackLayoutSpecStyle stackLayoutStyle;
-    
-    ASStackTextLayoutBaselineAlignment baselineAlignment;
+  /** Describes how the stack will be laid out */
+  ASStackLayoutSpecStyle stackLayoutStyle;
+  
+  /** The type of baseline alignment */
+  ASStackTextLayoutBaselineAlignment baselineAlignment;
 } ASStackTextLayoutSpecStyle;
 
-@interface ASStackTextLayoutSpec : ASLayoutSpec
+
+/**
+ A specialized version of a stack layout that aligns its children on a baseline. This spec only works with
+ ASStackTextLayoutable children. 
+ 
+ If the spec is created with a horizontal direction, the children will be laid on a common baseline.
+ If the spec is created with a vertical direction, a child's vertical spacing will be measured from its
+ baseline instead of from the child's bounding box.
+*/
+@interface ASStackTextLayoutSpec : ASLayoutSpec <ASStackTextLayoutable>
 
 /**
  @param style Specifies how children are laid out.
- @param children ASLayoutable children to be positioned.
+ @param children ASTextLayoutable children to be positioned.
  */
 + (instancetype)newWithStyle:(ASStackTextLayoutSpecStyle)style children:(NSArray *)children;